German document no. DE 10 2004 046 086 A1 recites a method and a device for controlling an internal combustion engine. In this context, starting from a comparison of a variable which characterizes the combustion process in at least one cylinder, at least one control variable is determined for controlling the combustion process, using a specified setpoint value. The control variable may be, for example, a trigger time or an air mass.
New Otto engine combustion methods, which are frequently also designated as gasoline HCCI methods (homogeneous charge compression ignition) or CAI methods (controlled auto-ignition), differ from the usual externally supplied ignition in that the ignition of the fuel is not triggered using externally supplied ignition, as for instance, by a spark plug, but is based on a mixture of a fuel and an introduced exhaust gas having an elevated temperature. Before the ignition, the formation of a homogeneous mixture takes place. This leads to a plurality of exothermic centers in the combustion chamber. That being the case, the combustion of the fuel takes place uniformly and rapidly. In addition, the combustion temperature in the HCCI method is below the combustion temperatures of the usual external auto-ignition methods. The new Otto engine combustion methods therefore stand out compared to the usual external auto-ignition methods because of lower fuel consumption and reduced untreated pollutant emission. One may do without a relatively costly exhaust gas aftertreatment system, such as the use of an NOx adsorption catalyst, in the HCCI method.
Self-igniting internal combustion engines, which are frequently also designated as CAI engines, are equipped with a variable valve operation and direct fuel injection, as a rule. With regard to the valve operation, the distinction is particularly made between fully variable and partially variable valve operations. A fully variable valve operation is ensured, for instance, by an EHVS (electrohydraulic valve control). One cost-effective alternative is represented by a partially variable valve operation, such as camshaft-controlled valve operation having 2-point valve lift and a phase shifter.
During operation of a self-igniting internal combustion engine, a relatively large residual gas quantity is retained in the cylinder, which takes care of the initiation of combustion during the compression phase. For example, using valve overlap, after the start of a combustion cycle, a certain residual gas quantity is able to be held in the cylinder for the following cycle. This is also referred to as a retained or an internal exhaust gas quantity for the following intermediate compression. Alternatively, the residual gas may also be returned externally or sucked back by the brief opening of the starting valve during the intake phase. In both cases, this is referred to as a returned or external exhaust gas quantity.
The combustion position of a self-igniting fuel combustion is frequently given using a crank angle. One would preferably designate thereby a specific energy turnover of the combustion. As an example, the combustion position is given as the combustion center point MFB50 (mass fraction burnt 50%).
However, in a self-igniting internal combustion engine, the direct trigger is lacking for initiating combustion, such as, for example, an ignition spark of a spark plug. For this reason, relatively large differences with respect to the course of the combustion may occur that are individual to each cylinder.
However, during combustion in a self-igniting internal combustion engine, deviations with respect to the combustion position that may occur must be only slight. If, for instance, combustion is too late, it becomes unstable, and may cause durable interruption in the combustion. On the other hand, if the combustion takes place too early, the exhaust gas for igniting the fuel in the following combustion cycle cools off too rapidly. In addition, these problems add up significantly if, for example, in one cylinder of an internal combustion engine the combustion sets in too early and in another cylinder of the internal combustion engine the combustion sets in too late. In addition, the untreated emission and the specific fuel consumption also demonstrate a strong, nonlinear dependence on the combustion position of a cylinder. This has the effect, for example, that the earliest cylinder emits more additional nitrogen oxides than are saved as a result of a later ignition of the latest cylinder.
It is therefore desirable, when operating a self-igniting internal combustion engine, to have available the possibility of reducing deviations (between the cylinders and to a specified setpoint value) with respect to the course of combustion.